Um I don’t know if you just mixed up the wording on your post, but it does sound like you might be doing the split bottom mount wrong. When you land it and it slides down the string, it should be hitting you throw hand finger not your non-throw hand finger. If it’s bouncing off the string on your throw hand then you’re either missing and hitting the wrong string, or you mean your non-throw hand.
With all that said, and you’ve made sure you know the proper way it’s done. The first response is 100% correct, PRACTICE & More Practice is the only way to get it. This was one of the harder mounts for me to get personally, so I can understand your frustration. I have short fingers (I think anyway) and it made learning and getting this down very hard in the beginning.
Only thing I can say is practice more then 1 mount in a throw. Don’t just try once, return and try again. Keep going. If you land it, dismount keeping your hands in the same place and remount. This will help build what people refer to as “muscle memory.”